Method for forming a colored oxide coating on surfaces of aluminum or aluminum alloy

ABSTRACT

COLORED OXIDE COATING HAVING A BROAD RANGE OF BRONZE COLORS ON ALUMINUM OR ALUMINUM ALLOY MATERIAL ARE FORMED BY ANODIC OXIDATION WITH AN AQUEOUS SOLUTION CONTAINING 0.5-10% BY WEIGHT PER VOLUME OF OXALIC ACID AND 0.05-1.00% BY WEIGHT PER VOLUME OF SULFURIC ACID. WHEN THE ELECTROLYTIC BATH IS MAINTAINED AT A LOW TEMPERATURE OR THE ABOVE DESCRIBED AQUEOUS SOLUTION CONTAINS ADDITIONALLY ALUMINUM ION, NITRIC ACID OR A MIXTURE OF NITRIC ACID AND BORIC ACID, THE DESCRIBED COLORED OXIDE COATING CAN BE OBTAINED IN A SHORTENED TIME.

United States Patent 3,836,439 METHOD FOR FORMING A COLORED OXIDECOATING ON SURFACES 0F ALUMINUM OR ALUMINUM ALLOY Masashi Ikegaya andFumio Shigeta, Shizuoka, Japan, assignors to Riken Light MetalIndustries Co., Ltd., Shizuoka, Japan No Drawing. Filed Oct. 20, 1972,Ser. No. 299,329 Claims priority, application Japan, Oct. 22, 1971,46/813,336, 46/83,.337, 46/83,338; Nov. 30, 1971, 46/95,850; May 12,1972, 47/46,487;

May 29, 1972, 47/52,528; July 5, 1972, 47/66,624

Int. Cl. C23b 9/02 US. Cl. 204-58 2 Claims ABSTRACT OF THE DISCLOSUREColored oxide coatings having a broad range of bronze colors on aluminumor aluminum alloy material are formed by anodic oxidation with anaqueous solution containing 05-10% by weight per volume of oxalic acidand ODS-1.0% by weight per volume of sulfuric acid. When theelectrolytic bath is maintained at a low temperature or the abovedescribed aqueous solution contains additionally aluminum ion, nitricacid or a mixture of nitric acid and boric acid, the described coloredoxide coating can be obtained in a shortened time.

The present invention relates to a method for forming a colored oxidecoating on surfaces of aluminum or aluminum alloy materials andparticularly a method for forming colored oxide coatings having a broadcolor tone of bronze series colors by subjecting aluminum or aluminumalloy materials to an anodic oxidation in an aqueous solution containingoxalic acid and sulfuric acid by means of direct current.

In the specification, aluminum or aluminum alloys are referred to asmerely aluminous materials and the direct current includes the rippleobtained by rectifying alternate current as well as current from anaccumulator.

Recently, aluminous building materials have been utilized broadly andvarious coloring processes for these materials have been studied.However, the range of coloration of the aluminous materials is not saidto be broader than that of the other building materials and forobtaining a certain color, for example, bronze, strict treatingconditions are required and further the color tone range is narrow andthe chemicals for such a treatment are usually expensive. Conventionalprocesses for coloring aluminous materials to bronze are as follows:

(1) An aqueous solution of oxalic acid is used as the electrolytic bath.

(2) Anodic oxidation is effected in an electrolytic bath containing anaromatic sulfonic acid and sulfuric acid or a metal sulfate.

In the above described first process, oxalic acid to be used in theelectrolytic bath is inexpensive and in this point this process is mostvaluable commercially. However, the color tone range of the obtainedoxide coatings is very narrow and even if the color tone is bronze, itbelongs to yellow series. Even if the quality of the aluminous materialitself or the thickness of oxide coating is varied, the color tone rangeis limited and consequently this process has not been extensively usedin practice.

The above described second process can provide a clear bronze color toneand a broad range and therefore this process has been used most broadly.However, the aromatic sulfonic acid which is the main component of theelectrolytic bath is expensive and this is a great drawback in thisprocess.

3,836,439 Patented Sept. 17, 1974 'ice Recently, such colored aluminousmaterials have been in great demand and in order to increase the yield,it is required to reduce the treating time but the above describedprocesses do not fully satisfy this requirement.

The inventors have made efforts to solve the above described drawbacksand accomplished the present invention.

The inventors have found that it is possible to obtain colored oxidecoatings having a broad range of bronze series color by a simpleelectrolytic bath, that is an aqueous solution of oxalic acid to whichsulfuric acid is only added.

An object of the present invention is to provide a method for formingeasily a colored oxide coating having a broad range of color tone ofbronze by means of an electrolytic bath of an aqueous solutioncontaining sulfuric acid in addition to oxalic acid.

A further object of the present invention is to provide a commerciallyadvantageous method for forming colored oxide coatings from bronze toamber in a short time by subjecting the aluminous materials to an anodicoxidation in an aqueous solution of oxalic acid added with sulfuric acidkept at a low temperature.

Another object of the present invention is to provide a method forforming colored oxide coatings from amber to bronze rapidly bysubjecting the aluminous materials to an anodic oxidation in an aqueoussolution of oxalic acid and sulfuric acid containing aluminum ion.

A further object of the present invention is to provide a method forforming stable colored oxide coatings from amber to bronze by subjectingthe aluminous materials to an anodic oxidation in an aqueous solution ofoxalic acid and sulfuric acid added with nitric acid or a mixture ofnitric acid and boric acid.

The invention will be explained hereinafter.

FIRST ASPECT OF THE PRESENT INVENTION For the practice of the method ofthe present invention, firstly, an aluminous material to be treated isdegreased with an aqueous solution of sodium hydroxide and then washedwith water and successively neutralized with an aqueous solution ofnitric acid and then washed with water. Then, the thus treated aluminousmaterial is subjected to anodic oxidation with an aqueous solutioncontaining 0.510.0% by weight per volume of oxalic acid and 0.05-1.0% byweight per volume of sulfuric acid.

When the amount of oxalic acid is less than 0.5% by weight per volume,the colored oxide coating is not fully formed and the mechanicalstrength is not sufficient and the color is not fully developed, whilewhen the amount of oxalic acid is more than 10% by weight per volume,oxalic acid is not dissolved in water at room temperature. Accordingly,the above limitation in the amount of oxalic acid is based on the abovedescribed reason.

When the amount of sulfuric acid is less than 0.05% by weight pervolume, the coloration of bronze is not sufiicient, while the upperlimit of sulfuric acid of 1.0% is based on the reason that an amount ofmore than 1.0% is not necessary for obtaining bronze color from therelation to the amount of oxalic acid.

The conditions of the anodic oxidation by using such an electrolyticbath are as follows:

(1) Current: direct current (2) Bath temperature: 0-40 C., preferably10-40 C. (3) Treating time: 5-120 minutes (4) Current density: 0.5-5a./dm.

Voltage: 5-150 v.

It seems that in the above described bath composition, oxalic acid andsulfuric acid contribute to the hue of the oxide coating and sulfuricacid mainly contributes to the value (luminosity).

However, under the above described conditions, when the content ofsulfuric acid in the electrolytic bath is less than 0.75% by weight pervolume, the content of oxalic acid is preferred to be more than 5.0% byweight per volume.

However, even when the content of sulfuric acid is less than 0.75% byweight per volume, if the content of oxalic acid is less than 5.0%, theobject of the present invention can be attained, but in this case unlessthe bath temperature and the current density are properly limited, thetreating time must be greatly prolonged in order to obtain asatisfactory colored oxide coating having a sufficient strength. Namely,if the bath temperature and the current density are 15:1" C. and 4-5a./dm. respectively, a bronze colored oxide coating having asatisfactory weather resistance can be obtained in about 20 minutes. Inthis case, even if the current density is less than 4 a./dm. when thetreating time is prolonged (for example, 1 hour), a bronze colored oxidecoating may be obtained but in order to shorten the treating time to 20minutes (at most 30 minutes), 4 a./dm. of current density is necessaryand even when the current density is increased to more than a./dm. theeffect of the increase of current density does not appear, so that theupper limit of the current density is 5 a./dm.

Too high a bath temperature is not preferred and it has been found thatthe bath temperature at which the desired colored oxide coating isobtained by the above described bath composition and current density inless than 20 minutes, is :1" C.

Furthermore, even under the above described conditions (1) to (4), whenthe content of sulfuric acid is more than 0.75% by weight per volume,the colored oxide coatings having the desired bronze color can beobtained within the range of oxalic acid of 05-10% by weight per volume.

In the above described electrolytic bath, the bronze coloration can beobtained in a very short time by increasing the current density, forexample, when the current density is 4 a./dm. the treating time is aboutminutes and if the current density is 5 a./dm. the treating time isabout 5 minutes.

SECOND ASPECT OF THE PRESENT INVENTION The inventors have further foundthat the desired color tone can be obtained independently of thethickness of the formed oxide coating layer by eifecting the anodicoxidation at a given lower bath temperature. That is, colored oxidecoatings from amber to bronze having various excellent properties can beobtained in a short time of 3-15 minutes by subjecting the aluminousmaterials to an anodic oxidation by means of an aqueous solutioncontaining (LS-5.0% by weight per volume of oxalic acid and ODS-0.50% byweight per volume of sulfuric acid at a low temperature of 0-12" C.under the same current density and voltage as described above.

In the above described treatment, the desired color tone can be obtainedby keeping the bath temperature low. In general, the color tone of theoxide coating is influenced by the thickness of said coating layer butas described above it has been found that the above described color tonecan be attained at the low bath temperature in the above described shorttime. The upper limit of the bath temperature is important and if thebath temperature is higher than 12 C., it is necessary to increase theelectric power per unit area, that is, to increase the thickness of theoxide coating in order to obtain a given coloration. For the purpose,the treating time is to be pronlonged and the object of the presentinvention for shortening the treating time can not be attained andtherefore the desired economic effect can not be attained. While, in thitreatment, the desired color tone can be attained without increasing thethickness of the oxide coating and particularly since the bathtemperature is low as described above, the amount of anode alumniurndissolved off is small and therefore it is possible to prevent the agingof the electrolytic bath and such a treatment is very advantageous alsoin this respect.

THIRD ASPECT OF THE PRESENT INVENTION The inventors have found that theanodic oxidation can be effected in a short time by using the abovedescribed electrolytic bath to which aluminum ion is intentionallyadded.

It has been previously proposed to eifect the anodic oxidation in thepresence of the dissolved aluminum ion. The purpose of the presence ofthe aluminum ion in this case is to prevent local corrosion caused byexcess current which flows on the aluminous materials upon the anodicoxidation treatment. The amount of aluminum dissolved 01f in this caseis, for example about 0.1-0.4 g./l. in an aqueous solution of oxalicacid. The upper limit is defined for the reason that the electriccorrosion on the whole surface can be prevented by such an amount andthe lower limit is defined for the reason that the elfect for preventingthe excess current can be attained from such an amount.

The inventors have found that aluminum ion in an aqueous solutioncontaining oxalic acid and sulfuric acid can reduce the treating timeand give uniform color tone in addition to the prevention of the abovedescribed excess current.

Furthermore, it is known that anodic oxidation for forming the coloredoxide coating is carried out in the presence of aluminum ion or thesalts thereof. However, the functions of the dissolved off aluminum ionin this known art are as follows:

(1) Aluminum ion itself gives the inherent function for generating colorin the oxide coating.

(2) Aluminum ion coprecipitates with colored oxide or hydroxide in theoxide coating to develop color.

Accordingly, in the above described prior art methods, it is verydifficult to obtain the colored oxide coating on the aluminous materialsin a short time.

The electrolytic bath to be used in this aspect of the present inventionis an aqueous solution containing 05-10% by weight per volume of oxalicacid, 0.05-l.0% by weight per volume of sulfuric acid and ODS-6.0 g./l.of aluminum ion.

Aluminum ion promotes the formation of colored oxide coating, shortensthe treating time and stabilizes the color tone of the oxide coatings.As mentioned above, aluminum ion generally has the effect of preventingexcess current but this is a negative effect. On the contrary, theaddition of a relatively large amount of aluminum ion in the presentinvention does not directly seek to prevent the excess current but toaccomplish the anodic oxidation even at a lower current density, whetherthe excess current is formed or not. Aluminum ion itself promotes thecoloration reaction and from this point of view the treating time alsocan be reduced and such a function can be attained by the addition of0.05-6.0 g./l. of aluminum ion in the electrolytic bath containingsulfuric acid in addition to oxalic acid.

The conditions for the anodic oxidation are as follows:

(a) Bath temperature: 0-40 C.

(b) Current density: 0.5-5 a./dm. (c) Voltrge: 30-80 v.

(d) Treating time: about 10 minutes.

The above described aluminum ion is obtained by the following means.

(1) Dissolving off the anode aluminum during the anodic oxidation step.

(2) Addition of aluminum oxalate or aluminum sulfate.

(3) immersing the aluminum anode in the electrolytic bath containingoxalic acid and sulfuric acid according to the present invention inwhich the temperature has been raised by continuing the anodicoxidation.

For obtaining aluminum ion, the combination of the above described means(1) and (3) is commercially advantageous, because the means (2) requiresparticular addition of the above described chemicals to the electrolyticbath and therefore in the following Examples 6 to 9, the aluminum ionwas formed by the combination of the above described means (1) and (3).

The conditions for the anodic oxidation in the above describedelectrolytic bath are as follows:

(a) Bath temperature: -40 C.

(b) Current density: 0.5-5 a./dm. (c) Voltage: 30-80 v.

(d) Treating time: about minutes.

The above described explanation is made with respect to the case wherethe aluminous materials, after being degreased and washed with water,are directly subjected to the anodic oxidation, but if a porous oxidecoating has been previously formed on the aluminous material byconventional anodic oxidation, the treating time can be reduced andfurther the color tone of the oxide coating can be stabilized.

FOURTH ASPECT OF THE PRESENT INVENTION The inventors have further foundthat when the electrolytic bath of an aqueous solution containing oxalicacid and sulfuric acid is supplied with nitric acid, the resulting bathcan provide colored oxide coatings having stable bronze series colors ina shorter time than the electrolytic bath containing oxalic acid andsulfuric acid.

The electrolytic bath in this treatment is an aqueous solutioncontaining 0.5-10.0% by weight per volume of oxalic acid, 0.05-1.0% byweight per volume of sulfuric acid and 0.05-0.5% by weight per volume ofnitric acid.

The addition of nitric acid lowers the electrical conductivity of theelectrolytic bath containing sulfuric acid and oxalic acid. Accordingly,when a given current density is applied to the electrolytic bathcontaining nitric acid in addition to oxalic acid and sulfuric acid, theincreased rate of voltage in said bath is earlier than that of theelectrolytic bath containing oxalic acid and sulfuric acid andconsequently colored oxide coating having a desired color tone can beobtained in a shorter time.

However, nitric acid is apt to cause electric corrosion on the aluminousmaterial during the anodic oxidation and nitric acid should not be addeduntil the electric corrosion occurs and therefore the upper limit ofnitric acid is defined in view of this point.

The lower limit of nitric acid is based on the reason that the effect ofadding nitric acid does not clearly appear in an amount less than 0.05%.

Furthermore, the inventors have found that the electrolytic bath of anaqueous solution containing 0.1-10.0 by weight per volume of boric acidin addition to oxalic acid, sulfuric acid and nitric acid can providecolored oxide coating having stable greyish green to brown series colorsby an anodic oxidation treatment in a further shorter treating time.

Boric acid also has the function of decreasing the electricalconductivity of the electrolytic bath, as in nitric acid, but it doesnot have the danger of the electrical corrosion. Accordingly, it hasbeen found that when boric acid is added to further decrease theelectrical conductivity, the time for the anodic oxidation can befurther reduced as compared with the case when only nitric acid isadded.

Furthermore, it has been found that the addition of boric acid can givea bluish bronze color to the oxide coating.

In this case the conditions for the anodic oxidation are as follows:

(a) Bath temperature: 10-40 C. (b) Current density: 0.5-5 a./dm. (c)Voltage: 15-120 v.

(d) Treating time: 3-120 minutes.

The treating time is sufiicient in about 3 minutes but in general 20-30minutes are preferable and in this case the preferred bath conditionsare as follows:

Oxalic acid: 1.5-3.5% by weight per volume Sulfuric acid: 0.05-0.3% byweight per volume Nitric acid: -0.05-0.2% by weight per volume Boricacid: 1.0-3.0% by weight per volume Bath temperature: 20- -2 C.

The following examples are given for the purpose of illustration of thisinvention and are not intended as limitations thereof. In the examples,percent by weight means percent by weight per volume.

EXAMPLE 1 Aluminum 1100 was immersed in 8.0% by weight of an aqueoussolution of sodium hydroxide at 75 C. for 20 seconds and washed withwater and then the thus degreased aluminous material was immersed in 16%by weight of an aqueous solution of nitric acid at room temperature for10 seconds to effect neutralization and it then was washed with water.

The thus pretreated aluminous material was anodized as an anode in anelectrolytic bath of an aqueous solution containing oxalic acid andsulfuric acid as shown in the following Table 1 at a temperature of 20ilC., under conditions of a direct current of a current density of 2a./dm. and a voltage of 30-85 v. for minutes to obtain colored oxidecoatings as shown in Table I.

TABLE 1 Composition of electrolytic bath (percent by weight) Thicknessof l t Sulfuric Voltage coating 00 or one Oxalic acid acid (v.) ColorMunsell notation 5.0 0. 05 45-65 29 5. 7 2. 1 0. 10 40-60 31 5. 4. 7/2.1 0. 30 35-50 30 Bronze 6. 3. 8/1. 7 0. 50 35-50 31 Slightly lightbronze. 4. 9/1. 3

7.5 5 55-70 29 Greyish ye1low...- 5.8 GY 5. 2 1. 8 0. 10 50-65 29 Bronze6.7 GY 4. 1/1. 8 0. 30 50-60 28 Slightly light bronze. 7.9 GY 4. 3/1. 40. 50 45-50 27 Slightly light greyish yellow 9.5 GY 5. 8/1. 3 0. 45-5530 o 9.8 GY 5. 9/1. 5

10.0 0. 5 50-65 31 Light bronze 8.3 G 5. 6 3. 1 0. 10 50-60 27 Greyishyellow 3.0 GY 5. 5/2. 0 0. 30 45-55 28 Light grayish yellow 6.5 GY 4.8/1. 5 0. 5 40-55 30 Slightly light bronze- 7.9 GY 5. 1/ 1. 3 0. 75 3 30Bronze 9.0 GY 6. 0/1. 1

The conditions of the anodic oxidation in this case are as follows:

(a) Bath temperature: 10-30 C.

(b) Current density: 0.5-5 a./dm. (c) Voltage: 15-80 v.

Furthermore, the most favorable bronze is obtained at 0.10% of sulfuricacid in the case of a level of 7.5% of oxalic acid and at 0.75% ofsulfuric acid in case of the level of 10.0% of oxalic acid,respectively. The colors obtained by varying the amount of sulfuric acidat each level of oxalic acid belong to the bronze series, which isdistinguished from yellow, as shown in the Munsell notation. In thisrespect, it is understood that a broad range of bronze series colors isobtained according to the greased aluminousmaterial was immersed in 15%by weight of an aqueous solution of nitric acid at room temperature for30 seconds, to effect neutralization and it was then washed with water.v

The thus pretreated aluminous material was anodized as an anode in anelectrolytic bath of an aqueous solution containing oxalic acid andsulfuric acid as shown in the following Table 4 at a temperature of201-1 0, with a direct current of a current density of 2 a./dm. and avoltage of 3085 v. for 60 minutes to obtain present invention. 10

EXAMPLE 2 colored oxide coatings as shown in Table 4.

Aluminum 1100 was immersed in 8% by weight of TABLE4 c an aqueoussolution of sodium hydroxide at 80 C. for Compositionelectrolytic 20seconds and washed with water and then the thus de- 15 both (percent byweight) Thickgreased aluminous material was immersed in 16% by SulfuricVoltage 232;; weight of an aqueous solution of nitric acid at room tem-Oxalic acid acid (v.) (a) Color perature for seconds to efi'ectneutralization and it then 1 075 6&8; 29 Bronze was washed with water. 3

The thus pretreated aluminous material was anodized 20 2&2; 33 Rg' as ananode in an electrolytic bath of an aqueous solu- 5 B tion containingoxalic acid and sulfuric acid as shown in $5. 2,2128 3? fi the followingTable 2 at a temperature of i1 C., with 7 5 0 a direct current of acurrent density of 4 a./dm. for 12$ 3 light bmme' minutes to obtaincolored oxide coatings as shown in 10.0 0.75 -45 30 Do. Table 2.

TABLE 2 Composition of electrolytic bath (percent by weight) Thicknessof Color tone Suliuric Voltage coating Oxalie acid acid (v.) (a) ColorMunseil n tati n 1.0 0.05 65-85 27 Light bronze 4.9 G 4.3/1.9 0.10 60-8028 Deep bronze 10 GY 3. 5/1.8

3.0 0.05 50-70 30 Light grayish yellow 2 0 GY 5. s/i. 5 0.10 45-60 28Greyish yellow 5.5/2.0 0.30 -55 31 Bronze 6.9 GY 3.7/1.7

5.0 0.05 -65 29 Greylsh yellow 2.5 GY 5.7/2. 1 0. 10 40-60 31 Lightbronze 5.0 G 4. 7/2. 1 0.30 35-50 30 Bronze- 6.5 GY 3.5/1.7 0. 35-50 31Deep bronz 7.5 GY 4. 9/1. 3

EXAMPLE 3 As mentioned above, the present invention is directed Twotests were effected in the same manner as described in Example 2, underthe following conditions as shown in Table 3, to obtain the results asshown in Table 3. In this case the voltage in the electrolytic baths wasnot measured.

As seen from the results of Table 3, bronze is obtained in a treatingtime of about 20 minutes only when the bath temperature is maintained at151-1 0., while if the bath temperature exceeds 15 :1" C., the treatingtime should be prolonged.

As seen from the above results, in this example the aluminous materialcan be colored in bronze for a comparatively short time under thedefined bath temperature and current density and therefore the cost ofchemicals used is remarkably decreased. Since the bath temperature is 15(2., the electrolysis is very easy. Furthermore, the oxide coatings thusobtained are very broad in color tone and have substantially the sameweather resistance as that obtained with the expensive aromatic sulfonicacid bath.

EXAMPLE 4 Aluminum 1100 was immersed in 8.0% by weight of an aqueoussolution of sodium hydroxide at 75 C. for 30 seconds and washed withwater and then the thus deto a method of obtaining bronze series colorsby using inexpensive oxalic acid and sulfuric acid and in this respect,it has high industrial and economic merit. Furthermore, the treatingtime can be shortened up to the order of 5 minutes when the currentdensity is raised, and consequently the amount of chemicals consumedlargely decreases and further the obtained coating has a suflicientweather resistance.

EXAMPLE 5 Aluminum 1100 was immersed in 8.0% by weight of an aqueoussolution of sodium hydroxide at 60 C. for 1 minute and washed with waterand then the thus degreased aluminous material was immersed in 16% byweight of nitric acid at room temperature for 20 seconds to effectneutralization and it was then washed with water. Then, the thuspretreated aluminous material was anodized as an anode in anelectrolytic bath of an aqueous solution containing oxalic acid andsulfuric acid as shown in the following Table 5 at a temperature of 1li1C., with a direct current of a current density of l a./d1n. and ahighest voltage of 70 v. for 3 minutes or 15 minutes to obtain coloredoxide coatings varying from bronze The sample obtained in this exampleand a comparative sample obtained by a conventional sulfuric acidelectrolytic bath were determined with respect to the followingproperties to obtain results as shown in the following Table 6.

TABLE 6 Comparative sample obtained by Sample plate of Test conventionalsulfuric acid bath (6 Example (3.5

1. 350 sec- 310 sec. 2- 60 sec 70 sec. 3- More than BN9 More than RNlO.

From the results of Table 6, it was confirmed that the properties of theoxide coatings according to the present invention are superior to thoseof the comparative sample even when the thickness of the coating isthinner.

As mentioned above, the present invention is directed to a method ofobtaining a color from bronze to amber in a very short time by usinginexpensive oxalic acid and sulfuric acid and in this respect, has ahigh industrial and economic merit.

Moreover, the obtained oxide coatings have a broad range of color toneand are the same as those obtained with the expensive aromatic sulfonicacid bath in weather resistance, abrasion resistance and the like.

EXAMPLE 6 Aluminum 1100 was degreased, washed with water and neutralizedin the same manner as described in Example 5 and then the thuspretreated aluminous material was anodized as an anode in anelectrolytic bath of an aqueous solution containing oxalic acid,sulfuric acid and aluminum ion as shown in the following Table 7 at atemperature of 20:2 C., with a direct current of a current density of 2a./dm. and a voltage of 48 v. for 5 minutes, 7 minutes or 10 minutes toobtain colored oxide coatings showing uniformly from amber to bronze onthe aluminous material as shown in Table 7.

The same aluminous material as used in Example 6 was degreased andwashed with water in the same manner as described in Example 5 and thenthe thus pretreated aluminous material was anodized as an anode in anelectrolytic bath of an aqueous solution containing by weight ofsulfuric acid in a conventional manner for 3 minutes to form a porousoxide coating on the surface thereof and thereafter it was washed withwater.

Then, the thus treated aluminous material was further anodized in thesame electrolytic bath containing alumi num ion under the sameconditions as used in Example 6 until the same colored oxide coating asobtained in Example 6 was formed, whereby colored oxide coatings showinguniformly from amber to bronze similar to those of Example 6 wereobtained on the aluminous materials for 4 minutes, 6 minutes and 8.5minutes, respectively. Accordingly, it was confirmed that when theconventional sulfuric acid electrolysis is carried out as anintermediate treatment for a very short time, the treating time fordeveloping the color can be shortened.

EXAMPLE 8 The same aluminous material as used in Example 6 wasdegreased, washed with water and neutralized in the same manner asdescribed in Example 5 and then the thus pretreated aluminous materialwas anodized as an anode in an electrolytic bath of an aqueous solutioncontaining 2.2% by weight of oxalic acid, 0.22% by weight of sulfuricacid and aluminum ion as shown in the following Table 8, under the sameconditions for the anodic oxidation as described in Example 6, for 7minutes to obtain colored oxide coatings showing uniformly from amber tobronze on the aluminous material as shown in Table 8. It was confirmedthat the concentration of aluminum ion dissolved has a relation to theincrease of electrical resistance and has a favorable influence on thecoloring of the aluminous material.

TABLE 8 Concentration of aluminum ion, g./l.: Color 0.6 Light amber. 2.0Amber. 3.5 Bronze.

EXAMPLE 9 TABLE 9 Thickness of coating Color Aluminous material t)(Munsell notation) Pure aluminum (99.99%) 7. 5 1.5 Y 4. 97/12. 0 60637.5 7.8 GY 4. 21/6 5 7. 8 7.5 GY 4.13/11 8 7.5 IOGY 4.90 .0

From the above description, it can be seen that according to the presentinvention, the presence of aluminum ion in the electrolytic bath of anaqueous solution of oxalic acid and sulfuric acid controls thedissolution of aluminum ion from the aluminous material during theanodic oxidation and prevents the local electrical corrosion of thealuminous material to increase the electric resistance and therefore thevoltage is effectively raised while maintaining a low current density,whereby the formation of the colored oxide coating can be considerablypromoted. Accordingly, uniform coloration which has been obtained withdifi'iculty in a conventional process, can be easily obtained and it ispossible to obtain the colored oxide coating commercially andpractically.

EXAMPLE 10 Aluminum 1100 was immersed in 8% by weight of an aqueoussolution of sodium hydroxide at 70 C. for 30 seconds and washed withwater and then the thus degreased aluminous material was immersed in 16%by weight of an aqueous solution of nitric acid at room temperature for10 seconds to effect neutralization and it was then Washed with water.Then, the thus pretreated aluminous material was anodized as an anode inan electrolytic bath of an aqueous solution containing oxalic acid,sulfuric acid and-nitric acid as shown in the following Table 10 at atemperature of 2012 C., with a direct current of of 20:2 C., with adirect current under the same electrolytic conditions as shown in Table11 to obtain colored oxide coatings of greyish green to greyish brown asshown a current density of 2 a./dm. and a voltage of 50-60 v. in Table11.

TABLE 11 Composition electrolytic bath (percent by Electrolyticcondition weight) Thick- Current uses 01 Color tone Sulfuric NitricBorio Time Voltage density coating Oxalic acid ac acid acid (min.) (v.)(aldmfl) (11) Color Munsell notation 2.0 0.1 0. 0.5 30 4770 2.0 14131111511 brown.-. 5.7 G 4. 4/1. 0. 1 0. 05 1.0 30 47-72 2. 0 d0 7. 1 G4. 2/1. 4 0.1 0.05 3.0 30 45-72 2.0 Deep bluish brown- 7.1 G 4.2/1. 4

3.0 0.15 0.1 0.5 30 42-53 2.0 17 Slightlyllght bronze-..-.- 1.1 G4.8/1.2 0.15 0.1 1.0 30 4H7 2.0 10 ronze 1.1 G 4.5/1.2 0.15 0.1 3.0 3040-60 2.0 16 Bluish brown" 2.2 G 4.2/1.2 0.20 0.1 0.5 30 40-52 2.0 16Bronze.- 1.8 G 4.4/1. 1 0. 0. 1 1. 0 -54 2. 0 16 -d0- 0. 7 G 4. 2/1. 40.20 0.1 3.0 30 38-55 2.0 15 Blulsh brown 0.5 G 4.0/1.2

for 30 minutes to obtain colored oxide coatings as shown in Table 10.

TABLE 10 Sulfuric acid] Thick- Oxaiic acid nitric acid ness of Colortone (percent by (percent by Voltage coating weight) weight) (v.) (0)Color Munsell notation 2 0. 05/0. 05 -60 14 Light grayish brown 7.3 GY5. 4/1.0 0 05/0.10 -60 16 Greyish brown..-. 8.1 GY 5.5/1.2 0.10/0.05 -6010 Brown 3.3 G 4.3/1.2 010/010 -60 15 Dark grayish brown" 9.7 CY 3.3/1.40.15/0.05 -60 17 Dark brown 9.5 GY 4.0/1.3

3 0. /0. -50 15 Greyish yellow 6.2 GY 5.6/1.2 0.10/0.10 -50 10 Lightbrown--- 6.3 GY 5.3/1.0 0. 10/0. 15 -50 15 Greylsh brown. 6. 5 GY 5.1/1. 2 0.15/0.05 -50 do 3.8 GY 4. 3/0. 9 0.15/0.10 -50 8.2 GY 4.0/1.20.15/0.15 -50 8.7 GY 4.9/1.4 0. 20/005 -50 9.1 GY 3.2/1.0 0. 20/0. 10-50 9.2 GY 3.4/0.9 0. 25/0. 05 -50 6.4 GY 3.9/1.3

6 0.05/02) -50 2.9 GY 5. 7/1.4 0.10/0.20 -50 2.7 GY 5.6/1.4 0.15/0.15-50 3.1 GY 4.9/1.5 0. 15/0. 20 -50 3. 1 GY 5. 2/ 1. 4 0.20/0. 15 -50 3.0GY 5.3/1.3 0. 20/020 -50 2.7 GY 5.5/1.7 0. 25/0. 10 -50 3.5 GY 5.2/1.20.25/0.15 -50 4.3 GY 5.1/0.9 0.25/0.20 -50 0 4.0 GY 5.0/0.9 030/005 -5014 Light grayish yellow.. 4.2 CY 5.2/1.1 0.30/0.10 -50 10 Greyishyellow.- 3.7 GY 5.4/1.3 0.30/0.l5 -50 14 d0 3.6 GY 5.6/1.4 0.30/0.20 -5017 --d0 2.9 GY 5.5/1.7 0.35/0. 05 -50 16 Light greyish yellow 3.3 GY 4.9/1. 3 0.35/0.10 -50 16 Greyish yellow.-. 3.8 GY 4.8/1.2 0.35/0.15 -5016 do 4.1 GY 5.0/0.0 0. 40/0. 05 -50 17 4. 2 GY 5. 1/1. 1 0. 40/0. 10-50 15 Light yellow 3. 7 GY 5.8/1. 5 0. /005 15 Light grayish yellow 3.8GY 4.9/1. 4

EXAMPLE 11 EXAMPLE 12 Aluminum 1100 was immersed in 8% by weight of anaqueous solution of sodium hydroxide at 70 C. for 30 seconds and washedwith water and then the thus degreased aluminous material was immersedin 16% by weight of an aqueous solution of nitric acid at roomtemperature for 10 seconds to eifect neutralization and it was thenwashed with water.

The thus pretreated aluminous material was anodized as an anode in anelectrolytic bath of an aqueous solution containing oxalic acid,sulfuric acid, nitric acid and boric 7 acid as shown in the followingTable 11 at a temperature The same aluminous material as used in Example11 '13 colored oxide coatings of greyish green to greyish brown in ashort time as shown in Table 12.

'From this fact it is considered that the voltage mainly contributes tocolor development.

of 12 C. for 3-15 minutes with a current density of 0.5- a./dm.

2. A method as claimed in claim 1, wherein said aqueous solutioncontains either 0.050.5% by weight TABLE 12 Composition of electrolyticbath (percent by weight) Electrolytic condition Color tone Thick- Sul-Current ness of Oxalic (uric Nitric Boric Time density coating acid acidacid acid (min.) Voltage (v.) (a./dm. 4) Color Munsell notation 2.0 0. 10. 05 1. 0 15 -1 min6 50 v.; -15 1. 8-4. 1 8. 0 Slightly light bronze 10G 5. 0/1. 3

n., v. 0.1 0.05 3.0 .'..do 1 65-3.0 8.0 do 6.8 G 4.8/1.7 0. 1 0.05 3. 010 -ni min.5 55 v.; 8-10 1 2-3.8 5.2 Greyish brown 7.5 G 4. 6/1. 5

n. v. i 0. 1 0.05 3.0 10 60-. 5.8 Bluish brown 4.7/1.0 0.1 0.05 3.0 1558-63 12.0 Greyish yellow 1.8 G 4.5/1.0 0. 1 0. 05 3.0 48-65 13.5 Darkgrayish purple 7.0 GY 4. 5/0. 2 0.1 0.05 3.0 20 50-65 8.5 do 2.4 G4.5/1.1

dizing the aluminum or aluminum alloy material in an 3 aqueous solutionof 0.55% by weight per volume of oxalic acid, 0.05-0.5% by weight pervolume of sulfuric acid and 0.05-6.0 g./l. of aluminum ion at atemperature per volume of nitric acid or a mixture of 0.05-0.5% by 20weight per volume of nitric acid and 0.1-10.0% by weight per volume ofboric acid.

References Cited UNITED STATES PATENTS 3,252,875 5/ 1966 Economy 204583,616,308 10/1971 Cooke et al 204-58 3,616,297 10/1971 Cooke et a1.204-58 3,597,338 8/ 1971 Matsuyama et a1 2045 8 JOHN H. MACK, PrimaryExaminer R. L. ANDREWS, Assistant Examiner

